This invention generally relates to gas-liquid contactors used in the removal of particulate matter and acidic gases from utility and industrial combustion gases. More particularly, this invention is directed to an open spray absorber having a spray tower that is equipped with an internal structure that enhances the efficiency of the contactor by reintroducing into the gas flowing through the tower any liquid that has cohered to the tower walls.
Gas-liquid contactors are widely used to remove substances such as gases and particulate matter from combustion or flue gases produced by utility and industrial plants. Often of particular concern are sulfur dioxide (SO2) and other acidic gases produced by the combustion of fossil fuels and various industrial operations. Such gases are known to be hazardous to the environment, and therefore their emission into the atmosphere is closely regulated by clean air statutes. The method by which acidic gases are removed from flue gases while flowing through a spray tower or other type of gas-liquid contactor is known as wet flue gas desulfurization (FGD).
The cleansing action produced by a gas-liquid contactor is generally derived from flowing the flue gas upwardly through a tower countercurrently to a descending liquid, which contacts the flue gas and absorbs the acidic gases and particulate matter. Wet flue gas desulfurization processes typically involve the use of calcium-based slurries or sodium-based or ammonia-based solutions. As used herein, a slurry is a mixture of solids and liquid in which the solids content can be any desired level, including the extreme condition in which the slurry is termed a moist solid. Examples of calcium-based slurries are limestone (calcium carbonate; CaCO3) slurries and hydrated lime (calcium hydroxide; Ca(OH)2) slurries formed by action of water on lime (calcium oxide; CaO). Such slurries react with the acidic gases to form slurries of sulfate and sulfite salts that can be collected for disposal or recycling. Intimate contact between the alkaline slurry and acidic gases present in the flue gases, such as sulfur dioxide, hydrogen chloride (HCl) and hydrogen fluoride (HF), result in the absorption of the gases by the slurry. Thereafter, the slurry can be accumulated in a tank.
A spray tower 10 is represented schematically in vertical and horizontal cross-section in FIGS. 1 and 2, respectively, for a gas-liquid contactor known as an open spray absorber. The tower 10 is generally an upright tubular-shaped structure having a wall 14 that defines a vertical passage. An inlet duct 12 serves to introduce combustion gases into the tower 10. Above the inlet duct 12 are multiple banks of spray nozzles 16 that each introduces a spray 18 of a cleansing liquid, such as one of the above-noted slurries or solutions, into the passage formed by the wall 14. The number of banks of nozzles 16 provided will vary in accordance with the requirements of a given application. Intimate contact between the spray 18 and the flue gases rising through the tower 10 results in a cleansing action, by which the liquid and the entrapped or reacted gases are collected at the bottom of the tower 10 in a tank (not shown). The cleansed gases that continue to rise through the tower 10 then typically pass through a mist eliminator (not shown), and thereafter are either heated or passed directly to the atmosphere.
The liquid introduced by the nozzles 16 is typically in the form of fine droplets, typically in the range of about 0.5 to 5 millimeters in diameter. As is evident from FIGS. 1 and 2, the spray 18 produced by a nozzle 16 within one bank overlaps the spray 18 from adjacent nozzles 16 of the same bank. The spray 18 from each nozzle 16 also overlaps the spray 18 emitted by nozzles 16 of lower banks as the spray 18 flows downwardly through the tower 10 under the influence of gravity. From FIG. 1, it can be seen that, even with multiple banks of nozzles 16, a true uniform slurry distribution in the tower 10 is not achieved due to wall effects. The sprays 18 from the nozzles 16 nearest the wall 14 impinge on the wall 14, such that the liquid flows downwardly on the surface of the wall 14 without contributing effectively to acid gas and particulate removal. Consequently, the spray concentration or density in an annular-shaped outer region 22 of the passage, shown generally as being between the outermost nozzles 16 and the wall 14, is lower than that in the central region 20 of the tower 10. Lower spray density in the outer region 22 results in low resistance to gas flow, such that the flue gases flow upward at relatively high velocities along the wall 14 of the tower 10. The combination of low spray concentration and higher gas velocity near the wall 14 results in a low liquid-to-gas ratio (L/G), high flue gas penetration, and a reduced absorber efficiency.
In view of the above, it can be appreciated that the efficiency of gas-liquid contactors of the type shown in FIGS. 1 and 2 are reduced by a nonuniform distribution of liquid within the spray tower, and that enhanced efficiencies could be achieved if a more uniform distribution were achieved or otherwise compensated for.
It is an object of this invention to provide a more efficient gas-liquid contactor for the removal of acidic gases and/or particulate matter from flue gases produced by utility and industrial facilities.
It is a further object of this invention that such a contactor is configured to redistribute liquid within the contactor in order to achieve enhanced absorption and removal of gases and particulate matter from flue gases.
It is another object of this invention that such a contactor is configured to reduce gas penetration along the walls of the contactor in order to achieve enhanced absorption and removal of gases and particulate matter from flue gases.
The present invention provides a gas-liquid contactor for removing gases and particulate matter from flue gases produced by processing operations of the type carried out in utility and industrial plants. The contactor is generally an open spray absorber having a spray tower whose walls form a passage within the tower. Flue gases are introduced into the tower through an inlet from which the flue gases flow vertically upward or downward through the passage. Disposed within the passage are heads for introducing a liquid into the passage such that the liquid contacts the flue gases. As used herein, the term xe2x80x9cliquidxe2x80x9d encompasses any of the slurries and solutions employed in the industry with gas-liquid contactors. A portion of the liquid introduced by the heads contacts the wall of the tower, such that the portion of the liquid flows downwardly along the wall. Finally, the tower is equipped with a deflecting device disposed on or near the wall for deflecting the portion of the liquid away from the wall, and thereafter reintroducing the portion of the liquid into the passage so as to contact the gases flowing through the passage. The deflecting device is also preferably configured to obstruct the flow of gases along the wall in order to reduce gas penetration at the wall and divert the gases toward the center of the passage where more efficient contact with the liquid is made. In a preferred embodiment, the deflecting device reintroduces the portion of the liquid deflected from the wall as droplets into the passage to promote the absorption efficiency of the reintroduced liquid.
From the above, it can be seen that a significant advantage of this invention is that the deflecting device serves to reintroduce into the gas stream any portion of the liquid adhering to the walls of the towerxe2x80x94liquid which otherwise would not contribute effectively to removal of gases and particulate matter from the flue gases. By also obstructing the flow of flue gases along the wall of the tower, the deflecting device greatly improves the liquid/gas ratio near the wall. As a result, gas penetration through the tower is significantly reduced and the overall efficiency of the gas-liquid contactor is significantly enhanced.
Other objects and advantages of this invention will be better appreciated from the following detailed description.